The present invention relates to a transmission system comprising a transmitter with a channel encoder for deriving encoded digital symbols encoded according to one out of a plurality of channel codes from source symbols, the transmitter further comprises transmit means for transmitting the encoded digital symbols via a transmission medium to a receiver, the receiver comprises receive means for receiving a signal representing the encoded digital symbols from the transmission medium, the receiver further comprises quantization means for deriving a quantized signal being quantized in a plurality of quantization levels from the signal representing the encoded digital symbols, and a decoder for deriving an estimate of the source symbols from the quantized signal.
The invention also relates to a receiver, a decoding arrangement and a decoding method.
A transmission system according to the preamble is known from U.S. Pat. No. 5,666,170.
Such transmission systems can be used for transmitting digital video signals or data signals which are represented by source symbols, via a transmission medium such as a satellite channel, a terrestrial channel or a CATV channel. In order to ensure a virtual error free transmission of the source symbols, channel coding is used. This channel coding often includes the use of a concatenated coding scheme. This concatenated coding scheme involves the encoding of the source symbols using an outer channel code, and encoding the output symbols of the outer encoder by an inner code. The outer code is often a block code such as a Reed-solomon code, and the inner code is often a convolutional code. It is also possible that between outer coding and inner coding interleaving is used in order to spread burst errors over time.
In the receiver, the signal representing the encoded digital symbols is quantized and converted into a digital signal in order to exploit the advantages of present day digital signal processing techniques. The quantized digital signal is applied to the decoder for the selected (inner) channel code which derives decoded symbols from its input signal. It is observed that the present invention is not limited to the case where concatenated coding is used, but is also applicable when only one single channel code with soft decision decoding is used.
The number of quantization steps to be used in said quantization should be selected adequately. A high number of quantization steps results in an increased complexity of the digital circuitry used in the decoder. A too low number of quantization steps results in a performance degradation of the decoder. Furthermore, this trade-off depends also on the used channel code.
It is an object of the present invention to provide a transmission system according to the preamble with quantization levels which are appropriate for all channel codes. To achieve said object, the present invention is characterized in that the receiver comprises quantization control means for setting a ratio between a signal level at the input of the quantization means and said quantization levels to a value being dependent on the channel code.
The present invention is based on the recognition that for each channel code a working point represented by the value of Eb/N0 can be defined for which the performance in the complete system is sufficient. To avoid unnecessary system-degradation, the quantization steps are optimized for a decoder operating at the working point. By setting the ratio between the signal level at the input of the quantization means and the quantization steps in dependence on the channel code, it is obtained that the quantization steps have optimum values for all channel codes on or near the working point.
An embodiment of the invention is characterized in that the quantizing means comprise a quantizer for quantizing the signal representing the encoded digital symbols independently from the channel code, and in that the quantization means comprise translation means for translating an output signal of the quantizer into a quantized signal having a ratio between a signal level at the input of the quantization means and said quantization levels to a value being dependent on the channel code.
This embodiment of the invention enables the use of standard analog to digital converters for performing a quantization independent of the channel code. Subsequently, the translation means translate the output signal of the quantizer into a signal that is quantized in dependence on the channel code.
A further embodiment of the invention is characterized in that the number of quantization levels at the output of the quantization means is equal to 16.
It has been shown by experiments that by applying the teaching of the present invention the number of quantization levels can be reduced to 16, without significant degradation of the performance of the decoder.